1. Field of the Invention
The present invention relates to an image processing apparatus, method, and medium storing a program, which generate image data processed to suppress information leakage.
2. Description of the Related Art
Conventionally, for the purpose of inhibiting or suppressing form documents, resident cards, and the like from being copied, such content is printed on paper called copy protection paper that has undergone special printing. When such copy protection paper is copied using, for example, a copying machine, a character string “Copy Inhibited” emerges on a copied material although such character string is difficult for the human eye to see on an original material. Then, such paper produces an effect of making one who copied that paper pause use of the copied material. Furthermore, since one already recognizes that a form document or the like is printed on copy protection paper, such paper produces an effect of suppressing or warning against copying itself.
However, such copy protection paper requires higher cost than normal paper. Also, only a character string set at the time of manufacture of copy protection paper can emerge, and that paper has limited uses, resulting in poor flexibility in terms of applications of use.
On the other hand, along with the progress of digitization of various contents, contents such as form documents and resident cards are similarly digitized. However, digitization of these form documents and resident cards is in transition, and a content created using an information processing apparatus is normally output onto paper using, for example, a printer at the time of use.
As such situation demands, a technique which generates copy protection paper, which was conventionally prepared in advance by printmaking, using an information processing apparatus and printer attracts a lot of attention (Japanese Patent Laid-Open No. 2001-197297). With this technique, upon printing out a content, image data called a tint block pattern is generated in addition to content data, and these data are output in a superimposed manner. The tint block pattern is sometimes called a copy-forgery-inhibited pattern. The tint block pattern seems like a simple pattern or background image for the human eye on an original material (a printed material output by a printer). However, when that original material is copied, a predetermined character string, image, or the like is visualized. This original material can give the same warning effect as in the aforementioned copy protection paper. This technique can be achieved due to the remarkable advances in printer performance.
When a tint block pattern created using an information processing apparatus is output to be superimposed on content data, since it can be output using normal print paper, a cost reduction can be attained compared to use of the copy protection paper, which is prepared in advance. Furthermore, since a tint block pattern can be generated upon printing out content data, a character string to be visualized upon copying an original material can be freely determined as well as the color and the like of the tint block pattern. Alternatively, an output date and time, information unique to a printer, and the like can advantageously be used as a tint block pattern.
As described above, when an original material is copied, a predetermined character string or the like which is unrecognizable before copying is visualized, and a tint block pattern achieves an effect of suppressing one from using that copied material. In order to achieve this effect, a tint block pattern to be generated basically includes a region where the same image as an original material also remains on a copied material and a region where an image which exists on the original material disappears on the copied material or is diluted compared to that on the region where the image remains. The tint block pattern including these two regions preferably has nearly equal densities on these two regions when it is printed out.
That is, the printout tint block pattern has to be configured so that a hidden character string to be visualized on a copied material is difficult to be macroscopically recognized by the human visual sense. An image region of this tint block pattern which is hidden on a printout material but appears to be visually recognizable by the human visual sense on a copied material obtained by copying that printout material will be referred to as a “latent image” hereinafter. Also, an image region which disappears after copying or has a lower density than that of the latent image visualized on the copied material will be referred to as a “background (or background image)” hereinafter for the descriptive purpose. The tint block pattern basically includes the latent image and background image.
The latent image is formed of dots concentrated in a predetermined region. By contrast, the background part is formed of dots dispersed in a predetermined region. By setting the nearly equal dot densities in these regions, the latent image part and background part can hardly be distinguished on the printout material of the tint block pattern.
FIG. 30 is a view showing the states of dots in these two image regions. As shown in FIG. 30, the tint block pattern is configured by a background part in which dots are formed at dispersed positions in a predetermined region, and a latent image part in which dots are formed at concentrated positions. The dots in these two regions can be generated by halftone dot processing or dither processing under different conditions. For example, when a tint block pattern is generated using the halftone dot processing, it is desired that halftone dot processing using a low LPI value is applied to a latent image part, and that using a high LPI value is applied to a background part. Furthermore, when a tint block pattern is generated using the dither processing, it is desired that dither processing using a dot-concentrated dither matrix is applied to a latent image part, and that using a dot-dispersed dither matrix is applied to a background part.
In general, a reader unit and image forming unit of a copying machine have a limit level of reproduction ability depending on the input resolution of reading small dots on a document and the output resolution of reproducing small dots. When dots of the background part of the tint block pattern are formed to be smaller than the limit level of dots that can be reproduced by the copying machine, and dots of the latent image part are formed to be larger than the limit level, the dots of the latent image part can be reproduced, but the smaller dots of the background part cannot be reproduced on a copied material. By using such characteristic, a latent image is visualized on a copied material obtained by copying the tint block pattern. An image which is visualized on a copied material will be referred to as a visible image hereinafter. Even when the background part is reproduced by copying, the same effect as that obtained when dots are not reproduced can be obtained as long as the latent image part on the copied material can be clearly recognized.
Images 31a and 31b in FIG. 31 show visualization of a latent image. FIG. 31 conceptually depicts that an image formed of concentrated dots is visualized on a copied material, but an image formed of dispersed dots is not reproduced on the copied material.
Note that the tint block pattern printing is not limited to the above configuration as long as a character string or the like can be reproduced on a copied material at a recognizable level. That is, when a character string or the like is designated as a background part, and is visualized as an outline character string on a copied material, the tint block pattern printing can achieve its object.
On the other hand, in addition to the tint block pattern printing, a system which embeds tracking information in a paper medium itself in addition to a main body image using an information embedding technique such as LVBC (Low Visibility Barcodes) has been proposed. According to this system, if information leakage by means of a paper medium has occurred, tracking information can be extracted by analyzing a leaked document. When one carelessly brings out a document, his or her responsibility can be pursued based on the tracking information, and a suppression effect can be expected. Furthermore, a system which embeds copy job control information that inhibits copying on a paper medium itself upon execution of printing has also been proposed. In this system, a corresponding printer extracts the copy job control information to detect permission or inhibition of copying upon execution of copying, thereby allowing control over continuation or abortion of copying for each page. A system which embeds password information and permitted user information as condition information in place of simple copy control used to determine permission or inhibition, and permits only a specific user to copy has also been proposed (Japanese Patent Laid-Open No. 2003-280469). Note that the tint block pattern and LVBC will be collectively referred to as security dots hereinafter.
However, in the related art, when an image processing apparatus inadvertently composites images on documents including security dots, the security dots may be broken. For example, assume that a certain document includes a tint block pattern, and another document includes LVBC data. When image data of these documents are composited, and composite image data is output, since the security dots of both the tint block pattern and LVBC are embedded, each others' security dots interfere with each other and are broken, and the effect of a security function as an output material is lost.